Geological setting and paleoecology of the Upper

Nether lands Journal of Geosciences —– Geologie en Mijnbouw | Page 1 of 16. doi: 10.1017/njg.2014.32

Geological setting and paleoecology of the Upper CretaceousBench 19 Marine Vertebrate Bonebed at Bentiaba, Angola

C. Strganac1,2,*, L.L. Jacobs1, M.J. Polcyn1, O. Mateus3,4, T.S. Myers1, J. Salminen5, S.R. May1,R. Araujo1,4, K.M. Ferguson1, A. Olımpio Gonçalves6, M.L. Morais6, A.S. Schulp7,8,9 &T. da Silva Tavares6,10

1 Roy M. Huffington Department of Earth Sciences, Southern Methodist University, Dallas, TX 75275, USA

2 Perot Museum of Nature and Science, Dallas, TX 75201, USA

3 GeoBioTec, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal

4 Museu da Lourinh~a, Rua Jo~ao Luis de Moura, 2530-157, Lourinh~a, Portugal

5 Department of Physics and Department of Geosciences and Geography, P.O. Box 64, 00014 University of Helsinki, Finland

6 Departamento de Geologia, Faculdade de Ciencas, Universidade Agostinho Neto, Avenida 4 de Fevereiro 7, Luanda, Angola

7 Natuurhistorisch Museum Maastricht, De Bosquetplein 6/7, 6211KJ Maastricht, the Netherlands

8 Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081HV Amsterdam, the Netherlands

9 Naturalis Biodiversity Center, Darwinweg 2 / PO Box 9517, 2300RA Leiden, the Netherlands

10 Universite de Bourgogne, Dijon, France

* Corresponding author. Email: [email protected]

Manuscript received: 1 June 2014, accepted: 23 September 2014

Abstract

The Bench 19 Bonebed at Bentiaba, Angola, is a unique concentration of marine vertebrates preserving six species of mosasaurs in sediments best correlated by

magnetostratigraphy to chron C32n.1n between 71.4 and 71.64 Ma. The bonebed formed at a paleolatitude near 24°S, with an Atlantic width at that latitude

approximating 2700 km, roughly half that of the current width. The locality lies on an uncharacteristically narrow continental shelf near transform faults that

controlled the coastal outline of Africa in the formation of the South Atlantic Ocean. Biostratigraphic change through the Bentiaba section indicates that the

accumulation occurred in an ecological time dimension within the 240 ky bin delimited by chron 32n.1n. The fauna occurs in a 10 m sand unit in the Mocuio

Formation with bones and partial skeletons concentrated in, but not limited to, the basal 1–2 m. The sediment entombing the fossils is an immature feldspathic

sand shown by detrital zircon ages to be derived from nearby granitic shield rocks. Specimens do not appear to have a strong preferred orientation and they are

not concentrated in a strand line. Stable oxygen isotope analysis of associated bivalve shells indicates a water temperature of 18.5°C. The bonebed is clearly

mixed with scattered dinosaur and pterosaur elements in amarine assemblage. Gut contents, scavengingmarks and associated shed shark teeth in the Bench 19

Fauna indicate biological association and attrition due to feeding activities. The ecological diversity of mosasaur species is shown by tooth and body-size

disparity and by d13C analysis of tooth enamel, which indicate a variety of foraging areas and dietary niches. The Bench 19 Fauna was formed in arid latitudes

along a coastal desert similar to that of modern Namibia on a narrow, tectonically controlled continental shelf, in shallow waters below wave base. The area was

used as a foraging ground for diverse species, including molluscivorus Globidens phosphaticus, small species expected near the coast, abundant Prognathodon

kianda, which fed on other mosasaurs at Bench 19, and species that may have been transient and opportunistic feeders in the area.

Keywords: mosasaur, paleoecology, continental shelf, detrital zircons, Africa, Cretaceous

Introduction

Marine sediments at Bentiaba, Angola, have produced one of

the most diverse assemblages of marine amniotes known from

the Cretaceous (Mateus et al., 2012; Fig. 1; Table 1), compara-

ble to the type Maastricht fauna of the Netherlands and

Belgium (Jagt, 2005) and the Maastrichtian beds of Morocco

(Bardet, 2012). The Bentiaba assemblage is of particular

1© Netherlands Journal of Geosciences Foundation 2014

importance because of the high richness of mosasaurs, a group

of extinct marine lizards, and other top predators that have

been interpreted as ref lecting bottom-up selection pressure

within a productive ocean ecosystem during the last 32 million

years of Cretaceous time (Polcyn et al., 2014). Bentiaba and

other vertebrate fossil localities along the southwest coast of

Africa ref lect the effects of the growing South Atlantic Ocean

on Late Cretaceous marine ecosystems. The Bench 19 Fauna

of southern Angola provides a time-constrained glimpse of

a marine vertebrate assemblage during the latter phase of this

history, placed within the broader context of South Atlantic

rif ting, sea level, paleoclimate and Late Cretaceous time. In

this study we use d13C values of mosasaur and elasmosaurid

plesiosaur tooth enamel to investigate niche differentiation

and feeding strategies of the Bench 19 Fauna, and we analyse

the geological setting and paleoenvironment of the Bench 19

Bonebed to understand better the biological and physical factors

that led to formation of the assemblage (Rogers & Kidwell, 2007).

Differential carbonate cementation in the Mocuio Formation

at Bentiaba creates an alternating pattern of more and less

resistant beds. The prominent, resistant beds (or benches) were

numbered consecutively beginning with the most basal bench

in the Mocuio Formation. Overlying Bench 19 in the strati-

graphic section are 10 m of fine sandstone, capped at about

138 m in the section by a series of thin lag deposits composed

mainly of comminuted fish bone, as well as numerous shark and

mosasaur teeth (Strganac et al., 2014; Fig. 2). Although fossil

remains of mosasaurs, plesiosaurs and marine turtles are distrib-

uted throughout the 10 m of the Mocuio Formation overlying

Bench 19, vertebrate specimens are particularly concentrated

in a 1–2 m thick zone directly overlying Bench 19, referred to

here as the Bench 19 Bonebed and Fauna.

The chronological context for Bentiaba is provided by a d13C

chemostratigraphic curve and magnetic polarity stratigraphy

constrained by an 84.66 1.5 Ma 40Ar/39Ar whole-rock radiomet-

ric date on a basalt of the Ombe Formation and by ammonite bio-

stratigraphy (Strganac et al., 2014). The Bench 19 Bonebed is

located near the top of the Bentiaba section, in chron C32n.1n

between 71.4 and 71.64 Ma (Husson et al., 2011). Water tem-

perature of 18.5°C for the Bench 19 Fauna was determined

from a d18O chemostratigraphic curve derived from marine

bivalves (Strganac et al., in press).

Abbreviations

ML – Museu da Lourinh~a, Portugal; MGUAN-PA and PA – Museu

de Geologia da Universidade Agostinho Neto, Luanda, Angola

(PaleoAngola Collection); SMU – Southern Methodist University,

Dallas, USA.

Methods

Locations on the ground of amniote fossils were marked by GPS,

recorded in the field and marked on the stratigraphic section.

Nearest-neighbour data were compiled from GPS data and

the long-axis orientation of the specimens was noted for those

that could be identified to taxon and were clearly in place.

Remains of bony fish and sharks are found throughout the

Mocuio Formation and were collected opportunistically. Precise

specimen locality data are on file at SMU. Skeletal completeness

and bone modification features were noted during specimen

preparation at SMU and ML.

A predicted paleolatitude for Bentiaba from the Ombe

Formation dated at 84.6 Ma 6 1.5 Ma (Strganac et al., 2014)

was determined from oriented paleomagnetic samples collected

within the measured section at Bentiaba. Results from three

basalt samples yield a mean magnetisation direction of declina-

tion (D) = 346.9° and inclination (I) = –42.4°, with a95 = 7.8°

(Fisher, 1953), which is different from the Earth’s present

magnetic field direction at the sampling site (D = 353.2° and

I = –56.3°). Paleolatitude (l) can be calculated by the simple

equation:

tanðIÞ ¼ 2tanðlÞObtained mean inclination from Ombe Formation basalts

yields a paleolatitude of 24.5°S for the Bentiaba locality. The

paleolatitude at 71 Ma was a few degrees lower because of the

northward drift of Africa.

The paleolatitudinal trace of Bentiaba from the initial open-

ing of the South Atlantic in the Early Cretaceous to its current

latitude is presented in Jacobs et al. (2011), determined using

PointTracker (Scotese, 2008). The width of the South Atlantic

Ocean at 71 Ma and 20°S was determined from Earthworks BV

(Reeves, 2014). The bathymetric image of the Angolan

Fig. 1. Location map of Bentiaba. Inset, lower right, shows location of

Angola in southwest Africa. Dashed line represents boundary between

the Congo Craton and Mesozoic basins, labelled in capital letters. Modified

from Strganac et al. (2014).

Netherlands Journal of Geosciences —– Geologie en Mijnbouw

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Table 1. Annotated vertebrate fauna list for Bench 19 Bonebed.

Taxon Elements Notes

Chondrichthyes

Elasmobranchii

Squalicorax pristodontus Numerous shed teeth Most common shark at Bentiaba; outnumbers

Odontaspidae indet. 10:1

Odontaspidae indet. Shed teeth

cf. Ganopristis sp.cf. Ganopristis sp. Two shed teeth Rare, only two shed teeth found

Brachyrhizodus sp.Brachyrhizodus sp. Isolated teeth and partial tooth batteries

Osteichthyes

cf. Eodiaphyodus sp.cf. Eodiaphyodus sp. Single fragmentary tooth plate

Enchodus sp. Fragmentary skeletons, skull fragments,

isolated vertebrae, teeth

Common

Testudines

Cheloniidae

Euclastes sp.Euclastes sp. Two partial skulls

Protostegidae

?Calcarichelys sp.?Calcarichelys sp. Single neural scute

Protostega sp.Protostega sp. Large disarticulated skeleton

Toxochelyidae

Toxochelys sp. Fragmentary specimen comprising hyoplastron,

peripheral and costal plates

Squamata

Mosasauridae

Globidens phosphaticus One partial skeleton with skull, two partial skulls with

fragmentary postcrania, numerous shed teeth

Second most common mosasaur at Bentiaba

Halisaurus n. sp. Multiple partial skeletons with skull and limb material,

shed teeth

Three partial skeletons found together as gut

content, adult and subadult individuals present

Mosasaurus sp. Rare isolated shed teeth of large morph (M. cf.

hoffmanni) and two partial skulls, vertebrae, isolated

quadrates from a small Mosasaurus sp.

Represented as both gut content (small morph only)

and isolated finds; may be ontogenetic stages

of a single taxon.

Phosphorosaurus sp. Partial frontal

Platecarpus’ ptychodon Two semi-articulated partial skeletons, shed teeth Represented as gut content and isolated finds;

this taxon was referred to Platecarpus by Arambourg

(1952), but considered new unnamed genus

Prognathodon kianda Multiple partial skeletons, numerous shed teeth Most common mosasaur at Bentiaba; subadult

specimen represented in gut content

Plesiosauria

Elasmosauridae

cf. Tuarangisaurus Four specimens representing pectoral and pelvic girdle

elements, propodials, cervical and pectoral vertebrae

Shed teeth are rare

Elasmosauridae indet. Girdle elements, semi-articulated limb material,

vertebrae and fragmentary cranial material

Shed teeth are rare

Pterosauria

Ornithocheiroidea indet. Long bones and long bone fragments

Dinosauria

Ornithopoda

Hadrosauroidea indet. Isolated pedal element

Ornithopoda indet. Isolated long bone

Nether lands Journal of Geosciences —– Geologie en Mijnbouw

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continental shelf discussed below was constructed from

http://cmtt.tori.org.tw/data/App_map/maplist.htm.

Sediment samples from the Bench 19 Bonebed were taken

from the field jacket containing a specimen of Prognathodon

kianda (PA 183) at the base of the Bench 19 Bonebed and

analysed for mineral composition, grain size and detrital

zircon ages. A second sample was taken from the jacket of a

Prognathodon sp. (PA 186) near the top of the unit. To determine

the mineralogy of the Bench 19 Bonebed matrix, a small amount of

material was powdered with a mortar and pestle and analysed at

SMU using a Rigaku III Ultima X-ray diffractometer with a step size

of 0.05° over a range of 2–60° 2u. Grain size was determined by

sonicating a matrix sample in deionised water to disaggregate it

and sieving the dried matrix to separate the size fractions.

A single sample of sandstone (MJP 2006A) was analysed

for detrital zircon geochronology to investigate sediment prov-

enance. This sample comprises sand from the Mocuio Formation

that was excavated from one of the plaster jackets containing

a Prognathodon kianda (PA 183). Detrital zircons were analysed

using laser ablation inductively coupled plasma mass spectrometry

(LA-ICPMS) at the Arizona LaserChron Center (www.laserchron.

org) using methods described by Gehrels et al. (2006, 2008).

Isolated teeth of mosasaurs and plesiosaurs collected from the

Bench 19 Fauna were identified by comparison with specimens

at the Shuler Museum of Paleontology at SMU and published lit-

erature (Bardet & Pereda Superbiola, 2002; Schulp et al., 2006,

2008; Polcyn et al., 2010; Araujo et al., submitted A; submitted

B). Tooth specimens were soaked in 10% acetic acid for at least

72 hours to remove diagenetic carbonate encrustation and

rinsed with deionised water. The samples were then sonicated,

treated with methanol, rinsed again in a deionised water bath

and dried. Enamel was sampled using a carbide-tipped hand drill

and the resulting enamel powders were placed in vacuum-sealed

reaction vessels with 100% orthophosphoric acid at 25°C for

a minimum of 4 hours. The evolved gas was cryogenically purified

to isolated CO2, which was analysed with a Finnegan MAT 252

isotope ratio mass spectrometer to determine its d13C and d18O

values, reported here in permil units relative to the Vienna

Pee-Dee Belemnite standard (VPDB). Data obtained for individ-

uals or groups of taxa were compared statistically using the

nonparametric Mann–Whitney U test for two groups of data

and Kruskal–Wallis ANOVA for three or more groups.

Results

Sedimentology and detrital zircons

The Mocuio Formation at Bentiaba is composed primarily of

massive fine-grained sandstone with, near the top, occasional

low-angle cross-bedding and some thin fossiliferous lag deposits.

Scour marks are minor and rare, and there is no evidence of

high-energy deposition or wave action, such as hummocky

cross-stratification, indicating that depth was below storm

wave base. The macroinvertebrate assemblage is relatively

sparse and comprises fragments of the pteriomorphian bivalve

Inoceramus, dissolution moulds of non-inoceramid bivalves

and irregular vertical burrows.

Although vertebrate fossils occur sporadically throughout

the upper Mocuio Formation, they are concentrated in a 1–2

m thick bonebed overlying Bench 19, although the Bench

19 Fauna (Table 1) occurs throughout the overlying 10 m.

The base of Bench 19 is planar and the overlying fossil-bearing

sediments grade upward rapidly into friable sand that forms

steep walls, overlain by more frequent, albeit uncommon,

cross-beds and fossiliferous lags. Terrestrial inf luence in Bench

19 and the overlying 10 m includes rare oxidised, unidentifiable

plant remains and occasional isolated dinosaur and pterosaur

bones (Mateus et al., 2012), indicating proximity to shore.

No pollen or microfossils were recovered.

The basal Bench 19 Bonebed sediment (sample from PA183;

Fig. 3A) comprises a pale yellow (2.5Y 8/2) to white (2.5Y 8/1)

poorly sorted, subangular to subrounded, silty, fine-grained

feldspathic arenite, weakly cemented by calcite. Clast size

0 m

100 m

50 m

Piambo

Moc

uio

Salin

asBa

ba

Ombe

conglom.

clay

silt

v.f. sa

nd

f. sand

m. sand

c. sa

nd

vc. sa

nd

110 m

19181716

141513

12

1110

9

87

654321

Bench

cover140 m

Moc

uio

Form

atio

n

LEGENDin situ vertebrate fossil

float vertebrate fossil

shark teeth

inoceramid bivalvenon-inoceramid bivalve

Fig. 2. Stratigraphic section at Bentiaba, with Mocuio Formation enlarged.

The Ombe Formation is a basalt dated at 84.6 6 1.5 Ma (Strganac et al.,

2014), within the Santonian. The overlying Baba and Mocuio formations

are Campanian–Maastrichtian.


4

ranges from coarse sand to silt-size, with a positively skewed

distribution. Larger clasts are lithics and quartz, and smaller

clasts are predominantly microcline and anorthite. There is an

indistinct and gradual facies change in the upper part of the

Bench 19 unit (sample from PA 186). This part of the unit com-

prises a well-sorted, subrounded to rounded, very fine-grained

yellow (2.5Y 7/6) to olive yellow (2.5Y 6/6) silty sand. XRD

analysis indicates it is also a feldspathic arenite with a relatively

high abundance of quartz (75–90%). This level has a greater

textural and compositional maturity relative to the lower part

of the Bench 19 Bonebed.

Uranium-lead ages were obtained from 76 zircons extracted

from a single sample from the Bench 19 Bonebed sandstone

with a total age distribution of 107 6 2 Ma to 2571 6 7 Ma.

A standard age probability distribution (Fig. 3B) shows major

grain populations at ∼1996 Ma and ∼1807 Ma, and a late

Precambrian to Cambrian population with an age peak at

∼515 Ma. Uranium concentrations for all grains is < 600 ppm.

As illustrated in Fig. 3C, some of the Paleoproterozoic grains

depart from the concordia, suggesting potential lead loss during

the Pan-African orogenic event.

Fossil spatial distribution and orientation

The Bench 19 Fauna was collected from a contiguous band of

badland outcrops within an area approximately 500 m north–

south by 200 m east–west, with additional specimens obtained

from sporadic outcrops extending some 300 m east (Fig. 4A).

The Bench 19 unit intersects the eroded surface along a trace

approximately 1.5–2 km in length, estimated by connecting

the fossil occurrences on a map. The dip of the strata approxi-

mates the surface of Bench 19 in some places and in others

the intersection is along steep faces. Fragments of pterosaur

bones were found throughout the interval, but more complete

bones were discovered by their cross-sections in steep outcrop

faces, which afforded them protection from deeper weathering

and erosion. Bony fish and sharks, although not systematically

collected, appear broadly distributed, but shark teeth, especially

Squalicorax (Antunes & Cappetta, 2002) are closely intermingled

with amniote skeletal remains. A histogram of amniote near-

est-neighbour distances shows a peak between 2.5 and 5 m

(n = 192, range = 0–35 m, �x = 8.5, s = 6.5; Fig. 4B). The closest

associations of amniotes (0 m) are interpreted as gut contents

because of the position of one individual within the gut area

of a presumed consumer and the etched condition of bones

and teeth in consumed individuals (see discussion below).

There is no preferred orientation of elements and all speci-

mens rest parallel to the bedding plane (Fig. 4C). The spatial

distribution of amniote specimens and their separation by

metres indicates a scattered distribution over the sea f loor,

lacking linear orientation as in a strand, or concentration by

currents or other agents into a tight mass. The fossils are not

concentrated in ravinements or on scoured surfaces. There is

no evidence of turbulence in the sediments or of transport in

the condition of the bone.

Fig. 3. Bench 19 sediments and zircon age analysis. A. SEM image of sedi-

ment samples from main bonebed layer above Bench 19 taken from PA183.

Large grains are quartz and small grains are feldspathic. B. Detrital zircon

age probability distribution (red) and histogram (blue) for sample

MJP2006A from the Bonebed sandstone in the Mocuio Formation. Age of

key probability peaks is shown on the figure. C. Concordia plot for detrital

zicrons from MJP2006A showing error ellipses from resulting 206Pb/238U

and 207Pb/235U values.


5

Bone modification

Amniote fossils occur as partially articulated skeletons, disar-

ticulated but associated partial skeletons and isolated elements.

All specimens within the bonebed, except those interpreted as

gut contents, exhibit a similar preservation style. Postburial

crushing, indicated by longitudinal cracks, is common, but fos-

sils appear more completely permineralised with less apparent

postburial crushing in the upper part of the Bench 19 Bonebed.

Irrespective of position within the Bench 19 Bonebed, pitting

and erosion of cortical bone is present only in juvenile plesiosaur

limbs. No bioencrustation or microboring was observed. Evidence

of transport (abrasion, rounding or polishing) is absent and bone

surfaces are generally pristine (Fig. 5A).

Most specimens show evidence of scavenging by sharks.

Bite marks and scratches most closely match the size and serra-

tion pattern of Squalicorax pristodontus teeth, displaying second-

ary lineations perpendicular to the main grooves (Schwimmer

et al., 1997; Shimada et al., 2010). Marks from shark scavenging

are common on limb elements and ribs but rare on vertebrae, irre-

spective of taxon (Fig. 5B).

Mosasaur-on-mosasaur predation or scavenging is present

in one specimen of Prognathodon kianda (PA 183), which com-

prises the skull, some proximal rib portions and the vertebral

column complete to the anterior caudals, but is missing the

limbs, girdles and most of the ribs. The skeleton shows bite marks

attributed to sharks and numerous Squalicorax pristodontus teeth

were found with the specimen. Partial skulls of ‘Platecarpus’ ptycho-

don, Mosasaurus sp. and an indeterminate mosasaurine were found

in the gut region of PA 183 (Fig. 5C). In these gut-content speci-

mens, the skull roof, snout and other areas thinly fleshed in life

are etched, and tooth crowns are dissolved above the gum line.

A second mass of bones exhibiting similar modification

to those seen in the gut area of PA 183 comprises the partial

skeletons of three Halisaurus specimens (PA 179) and a juvenile

P. kianda (PA 25). Individual bones show no modification by

sharks. At the time of discovery and excavation, the mass was

thought to be isolated and a consumer remains undiscovered,

possibly still in the quarry. The size of the bone mass, if in fact

it is gut contents, suggests the consumer was a large mosasaur.

Results of d13C

The 49 enamel carbonate d13C values derived from 37 tooth

specimens of five mosasaur taxa and four plesiosaur teeth from

Bench 19 Bonebed are presented in Table 2 and Fig. 6. PA 314 (Hal-

isaurus) yielded a d13C value of –7.2‰. Specimens PA 177 and PA

312 (‘P.’ ptychodon) produced an average d13C value of –7.3‰.

Twenty-one teeth assigned to Prognathodon kianda produced an

average d13C value of –10.1‰ and ranged from –14.1 to –5.2‰.

Three teeth of Mosasaurus sp. yielded an average d13C value of

11.6‰ and ranged from –12.9 to –10.2‰. Eleven Globidens phos-

phaticus teeth yielded an average d13C value of 13.0‰ and ranged

from –16.1 to –10.8‰. Four plesiosaur teeth produced an average

d13C of –13.1‰ and ranged from –14.0 to –12.2‰.

Halisaurus and ‘P.’ ptychodon are grouped together for analy-

ses due to their small body size (<5 m in length; Polcyn et al.,

2014) and their similar d13C values. The standard deviation of

Halisaurus + ‘P.’ ptychodon (–7.3 6 0.4) does not overlap with

those of Prognathodon (–10.3 6 2.4), Globidens (–13.1 6 1.6)

or plesiosaurs (–13.1 6 0.7) and is therefore statistically

different.

Fig. 4. A. Satellite image of Bentiaba (map data: Google, DigitalGlobe). Red

dots represent locations of specimens recovered from Bench 19 Bonebed. B. His-

togram of nearest-neighbour distances among fossils at Bench 19. C. Rose dia-

gram for orientations of elongate fossil elements at Bench 19.


6

Mann–Whitney U tests (Table 3) were used to compare d13C

values between two taxa or groups at significance levels of 0.05.

Globidens values are significantly different from Prognathodon

(p = 0.001), but not from plesiosaurs (p = 0.862) or Mosasaurus

(p = 0.223). Mosasaurus is not significantly different from

Prognathodon (p = 0.316) or plesiosaurs (p = 0.114). The d13C

values of Prognathodon are not significantly different from

‘P.’ ptychodon + Halisaurus (p = 0.046).

In comparing the d13C values between more than two taxa,

Kruskal–Wallis ANOVA tests indicate Globidens, Mosasaurus

and plesiosaurs were not significantly different (p = 0.293).

The group is significantly different when Prognathodon is

included (p = 0.003). There is a significant difference in d13C

values derived from Mosasaurus, Prognathodon and plesiosaurs

(p = 0.046), and Globidens, Prognathodon and plesiosaurs

(p = 0.001).

Discussion

The modern marine ecosystem off the shore of southern

Angola and Namibia is highly productive due to the Benguela

Current and coastal upwelling system (Shannon & Nelson, 1996).

The Benguela upwelling zone provides nutrient-rich waters that

support a diverse marine ecosystem containing ∼70% of the

marine mammal species in the South Atlantic Ocean (Cury

et al., 2000). Although the modern Benguela upwelling system

is purported to have originated in the late Miocene (Diester-Haas

et al., 2002), the presence of organic-rich Cenomanian–Turonian

Fig. 5. A. Bite marks attributed to the shark Squalicorax pristodontus on a mosasaur rib from the Bench 19 Bonebed, enlarged in B. Note short transverse

marks along major groove from serrations on teeth. C. Specimen PA183, Prognathodon kianda, with preserved gut contents. Two specimens in the gut are

shown as highlighted regions 1 and 2, and are enlarged in D and E, respectively. A third specimen was recovered between the blocks and is not shown.


7

Table 2. d13C values of tooth enamel carbonate in Bench 19 mosasaurs and plesiosaurs. Length estimates from Polcyn et al. (2014).

Taxon Length (m) Specimen # d13C d18O

Globidens 4-6 PA 04 -14.6 -2.1

PA 05 -13.2 -1.7

PA 29 -14.3 -1.2

PA 30 -10.8 -1.3

PA 301 -12.6 -1.1

PA 307 -11.6 -1.7

PA 31 -12.4 -1.1

PA 313 -12.5 -1.5

PA 33 -11 -2.4

PA 500 -16.1 -2.1

PA 61 -14.3 -1.6

Halisaurus 3 PA 314 -7.2 -2.2

Mosasaurus 17 PA 309r1 -10.2 -1.4

PA 309r2 -8.9 -1.4

PA 44 -11.6 -2.6

PA 46 -12.9 -1.9

“P.” ptychodon 1.5 PA 177 -7.2 -2.1

PA 312r1 -7.2 -2.1

PA 312r2 -7.6 -2.4

Prognathodon 7-9 PA 177r1 -9.2 -2.6

PA 177r2 -9.2 -3.2

PA 321 -8.2 -2.3

PA 38 -10.9 -1.5

PA 55 -8.7 -3.6

PA 173 -9.1 -1.7

PA 173 -9.2 -2.3

PA 28r1 -11.1 -1.6

PA 28r2 -11.4 -2.3

PA 304r1 -6.9 -1.8

PA 304r2 -7.3 -1.5

PA 306 -12.1 -2.4

PA 315 -10.1 -1.8

PA 318 -12.4 -2.4

PA 35 -10.7 -1.5

PA 40 -11 -1.7

PA 41 -5.2 -1.9

PA 45 -10.4 -3.3

PA 47 -13.7 -2.4

PA 48 -10.9 -1.6

PA 50r1 -14.1 -3.7

PA 50r2 -13.4 -1.9

PA 53 -12.8 -3.5

PA 56 -6.6 -2

Plesiosauria PA 173 -12.2 -2.5


8

deposits offshore in western Africa and the rich marine verte-

brate fauna preserved in Turonian, Campanian and Maastrichtian

coastal outcrops of Angola indicate that latitudinally controlled,

high-productivity environments consistent with upwelling

existed in the Late Cretaceous (Handoh et al., 1999; Jacobs

et al., 2006, 2009; Polcyn et al., 2014).

The paleolatitude of Bentiaba, at the time of the accumula-

tion of the Bench 19 Fauna (C32n.1n; 71.40–71.64, using the

calibration of Husson et al., 2011; see Strganac et al., 2014)

was not more than the –24° as calculated for the Ombe Forma-

tion basalt (84.6 61.5 Ma 40Ar/39Ar whole-rock radiometric

date; Strganac et al., 2014) and as reported by Jacobs et al.

(2011). The width of the South Atlantic at paleolatitude 20°S

and 71 Ma was approximately 2700 km calculated from Reeves

(2014), roughly half the current width. Thus, the Bench 19

Fauna was emplaced at a paleolatitude analogous to the posi-

tion of Walvis Bay in the present-day latitudinal position of

Africa, although in a wide but narrower than present-day

ocean. This latitude is well within the area of the high-pressure,

descending limb of the southern Hadley cell, driving rich

upwelling along the coastal desert. Oceanic upwelling cells

provide nutrients that enhance primary productivity, which has

been suggested as a driver of mosasaur evolution generally

(Jacobs et al., 2009; Polcyn et al., 2014). The presence of abun-

dant marine amniote fossils in coastal Angola, specifically at

Bentiaba, is consistent with that hypothesis.

The continental shelf at Bentiaba is much narrower than in

northern Namibia (100–160 km in Walvis Bay, as opposed to

<10 km at Bentiaba; Fig. 7). The extremely narrow width of

the continental shelf and the orientation of the continental

margin in the Bentiaba region are controlled by the tectonics

of the Benguela and Lucapa transform faults, relating to the

opening of the South Atlantic (as discussed by Guiraud et al.,

2010; see also Moulin et al., 2012). The depth of the shelf near

Bentiaba was shallow throughout the Late Cretaceous (Strganac

et al., 2014). The 84.6 Ma Ombe Formation basalt f lowed across

the coastline, but based on paleomagnetic and d13C chemostrati-

graphic correlations (Strganac et al., 2014), the Bentiaba coast

was emergent as indicated by the missing section shown by 5

my of time not represented by strata subjacent to the Ombe For-

mation and 6 my of missing strata superjacent to a thin veneer of

sediment atop the Ombe Formation. Examination of sea-level

curves presented by Muller et al. (2008) and Miller et al.

(2011) does not suggest an apparent link to eustatic sea-level

fall as the cause for these hiatuses, but tectonically induced

stratigraphic hiatuses have been documented in the Benguela

basin to the north (Giraud et al., 2010). At Bentiaba, the two

Late Cretaceous episodes of relative sea-level fall were caused

by local to regional uplift. The intervening Ombe Formation

defines sea level at the time by crossing the Santonian shore-

line. The fossiliferous upper Campanian to lower Maastrichtian

Baba Formation, which overlies the unconformity, and the

Mocuio Formation, which contains the Bench 19 Fauna, repre-

sent shelf f looding resulting from relaxation and subsidence.

The narrow shallow shelf may have served to concentrate prey

items and attract predators in a relatively small area compared

to the shelf to the north and south. Cenozoic faulting and eu-

static sea level change culminated in a Pliocene or younger ter-

race that truncates Cretaceous deposits at Bentiaba, perhaps

comparable to terraces seen farther south in the Namibe basin

(Sessa et al., 2013).

The bonebed sandstone is compositionally immature and

texturally submature. Its poor sorting, incomplete rounding

and feldspathic composition indicate close proximity to a granitic

source. The suite of ages exhibited by detrital zircons (Fig. 3B)

show significant populations at 1996 Ma and 1807 Ma with lesser

populations at 515 and ∼2500 Ma. The observed age probability

distribution is consistent with a nearby source terrain for

the Mocuio Formation sandstones. Within close proximity to

the present-day outcrops, Angolan basement yields radiometric

ages of 1795–2243 Ma (Hanson, 2003). Angolan basement is

dominated by Paleoproterozoic granitic and gneissic lithologies

with inliers of Archean crust consistent with the dominance

of Paleoproterozoic detrital zircon ages (62 of the 76 grains

studied) and occasional Archean grains. Granitoid rocks of

the Pan-African Kaoko Belt, south of Bentiaba, yield

Table 2. Continued.

Taxon Length (m) Specimen # d13C d18O

PA 305 -12.5 -1.5

PA 305 -13.5 -2

PA 320 -14 -1

PA 36 -12.7 -2.7

PA 36 -13.7 -2.5

Fig. 6. d13C values of mosasaurs and plesiosaurs at Bench 19.


9

radiometric ages of 505–656 Ma (Hanson, 2003; Goscombe

et al., 2005). The youngest detrital zircon grains at 107 Ma

and 132 Ma are significantly older than the depositional age

of the Mocuio Formation. They may represent minor sediment

derivation from Early Cretaceous kimberlites and carbonatites

emplaced along the Lucapa fault zone (Giuraud et al., 2010).

While not providing a unique fingerprint of sediment prove-

nance, detrital zircon ages from the bonebed sandstone are

consistent with derivation from nearby basement outcrops

and do not require long distance sediment transport. The de-

trital zircon data show that the Precambrian basement was ex-

humed by the Cretaceous and that Bench 19 sedimentary rocks

could have been derived from a nearby source area. Thus, miner-

alogy, physical characteristics of grains, detrital zircon ages

and proximity of the granitic shield to the coast at Bentiaba

all indicate sediment transport by short intermittent or sea-

sonal rivers draining the shield and f lowing directly to the

sea. This is consistent with the sedimentation in the arid envi-

ronment seen today in northern Namibia (Calvert & Price,

1983). However, in northern Namibia, a 70-km band of or-

ganic-rich diatom ooze is deposited at depths shallower than

140 m (Calvert & Price, 1983) and along the shelf where terrig-

enous input from the coastal arid region is insufficient to di-

lute planktonic and skeletal organic matter. At Bentiaba,

there is no evidence of diatom ooze.

The Bench 19 unit was deposited relatively near the shore

based on the paleogeography of the Bentiaba coast, as shown

by the shoreline observed in the Ombe Formation, the narrow

shelf, and the composition and texture of the sediment. On

the other hand, it is interpreted to have been deposited below

storm wave base (Peters & Loss, 2012) because sedimentary

structures such as hummocky cross-stratification and signifi-

cant scour surfaces are absent. Furthermore, there is no evi-

dence of postmortem, pre-burial, subaerial weathering on

the bones of the marine species (Behrensmeyer, 1978), indi-

cating that the life, death and burial of these animals was

completely below the surface of the ocean. While the Bench

19 Bonebed preserves a range of completeness of specimens,

none appears to have bloated and f loated from gases released

during decomposition. The presence of relatively complete car-

casses in the bonebed suggests that the volume of gas gener-

ated and captured in the carcasses was insufficient to

overcome hydrostatic pressure, leaving the carcasses on the

bottom. Reisdorf et al. (2012), in their study of ichthyosaur ta-

phonomy and bathymetry, concluded that intact articulated car-

casses remaining on the sea bed due to hydrostatic pressure at

certain localities were indicative of water depths >50 m because

putrifying carcasses sometimes rise from depths of 50 m but do

not rise from depths of 100 m or more. Variability in the tendency

to rise, especially at shallower depths, is temperature dependent

due to the solubility of gases. The water temperature at Bentiaba

immediately below Bench 19 as determined from d18O of Inocer-

amus shells was 18.5°C (Strganac et al., in press). Assuming

a maximum shelf depth of 150 m, a reasonable range of depths

for the formation the Bench 19 Bonebed is below wave base, be-

tween 50 and 150 m.

The amniote fauna of the Bench 19 Bonebed was reviewed by

Mateus et al. (2012) and an updated, annotated faunal list is

provided in Table 1. Schulp et al. (2013) documented the pres-

ence of a large Prognathodon sp. and Carinodens sp., neither of

which is known from the Bench 19 Fauna, but rather from

higher stratigraphic levels. Lower in the section, in the Baba

Table 3. Results for statistical analysis of variance for marine amniote d13C values.

Test Independent variables P value Significance level

Mann-Whitney U Globidens Plesiosauria 0.862 0.05

Mann-Whitney U Globidens Mosasaurus 0.223 0.05

Mann-Whitney U Globidens Prognathodon 0.001 0.05

Mann-Whitney U Plesiosauria Mosasaurus 0.114 0.05

Mann-Whitney U Plesiosauria Prognathodon 0.021 0.05

Mann-Whitney U Prognathodon small 0.046 0.05

Mann-Whitney U Prognathodon Mosasaurus 0.316 0.05

Kruskal-Wallis ANOVA Globidens Mosasaurus 0.293 0.05

Plesiosauria

Kruskal-Wallis ANOVA Globidens Plesiosauria 0.003 0.05

Mosasaurus Prognathodon

Kruskal-Wallis ANOVA Plesiosauria Prognathodon 0.046 0.05

Mosasaurus

Kruskal-Wallis ANOVA Plesiosauria Prognathodon 0.001 0.05

Globidens


10

Formation, below the Bench 19 unit, a small russellosaurine mo-

sasaur (sensu Polcyn & Bell, 2005) is found. Russellosaurina is

the clade that includes the subfamilies Plioplatecarpinae and

Tylosaurinae, representatives of which are found in the Bench

19 Fauna, but the Baba specimen is distinct from those found

higher in the section. Thus, the amniote fauna from the Bench

19 Bonebed is taxonomically consistent but distinct taxonomi-

cally from levels above and below Bench 19. In addition, above

the Bench 19, the bivalve Inoceramus, common in the bonebed,

has its last appearance in the section, nurse sharks (Ginglymos-

tomatidae) have their first appearance and Squalicorax teeth ob-

tain a larger maximum size co-occurring with smaller forms. The

taxonomic internal consistency of the Bench 19 Fauna and the

short time bin into which it fits, compared to the fauna above

and below, suggests accumulation of the Bench 19 Bonebed on

an ecological rather than a geological timescale.

The fossiliferous fine-grained sandstone Bench 19 Bonebed

in the upper Mocuio Formation preserves an attritional fossil

assemblage, as indicated by the stratigraphic distribution of

specimens and a faunal list that includes pterosaurs and dino-

saurs as rare and isolated elements among an overwhelmingly

marine fauna. Attritional accumulations forming bonebeds

have an excess of bones relative to sediment input, and hence

exhibit some amount of time averaging. There is no evidence

of past exhumation and reburial of individual fossils within

the Bench 19 Bonebed, consequently the stratigraphic posi-

tion of fossils within the bonebed is a time-ordered sequence

and the fauna must be considered time-averaged in the strictest

sense, of a relatively short duration. Temporal limits on the du-

ration of the Bench 19 Bonebed are set by the length of chron

C32n.1n, which is the 240 ky bin between 71.4 and 71.64 Ma

(Strganac et al., 2014).

The Bench 19 Fauna does not appear to be a catastrophic

death assemblage such as reported in marine mammal strand-

ings caused by harmful algal blooms (Brongersma-Sanders,

1948, 1957; Brongersma-Sanders et al., 1980; Pyenson et al.,

2014) because at Bentiaba individuals do not align with strand,

the bones are not limited to a single plane and there is defini-

tive evidence of active predation or scavenging. Pyenson et al.

(2009) studied the formation of the Miocene Shark Tooth Hill

Bonebed of California and concluded that this rich accumulation,

which is both larger and denser than the Bench 19 Bonebed,

was a winnowed, attritional accumulation formed under

sediment-starved conditions and terminated when sediment

input increased. The time averaged in the Shark Tooth Hill

Bonebed is as much as 700 ky. At Bentiaba there is no evidence

of significant winnowing and mixing by bottom currents, large

specimens are partially articulated and scavenged, and biological

interactions related to feeding are clear.

Bench 19 does not fit the model of preservation in a subaque-

ous gravity f low as presented by Adams (2009) because the shelf

setting of Bentiaba is inconsistent with the geological setting

and depositional environment ref lected in sedimentary textures

and structures expected of active margins. However, the Pisco

Formation along the coast of western South America in lati-

tudes comparable to the paleolatitude of Bentiaba is also on

an active margin, yet preserves a diverse cetacean fauna, in-

cluding articulated skeletons with baleen. Sediments are both

diatomaceous and tuffaceous, and most fossil occurrences ap-

pear to be rapidly buried falls (Esperante et al., 2002, 2008;

Brand et al., 2004, 2011) or strandings (Pyenson et al.,

2014) in embayments. Along passive margins, Calvert Cliffs,

Maryland, preserves a rich cetacean fauna, but the accumula-

tion is attritional over millions of years (Vogt & Eshelman,

1987; Gottfried et al., 1994). In Africa, the Cretaceous upwell-

ing area along Morocco is the antitropical equivalent of that

which occurred at the same time along the Bentiaba coast

and it has produced a rich and varied marine fauna (Bardet

et al., 2010; Bardet, 2012). However, upwelling in Morocco

Fig. 7. Bathymetric map of southwest Africa. Arrow points to Bentiaba. Dashed

lines indicate fracture zones (FZ), which can be traced to transform faults at the

Mid-Atlantic Ridge. South of the Walvis Ridge is Walvis Bay. Note the width of the

continental shelf at Bentiaba compared to Walvis Bay. In the early Maastrichtian

(Late Cretaceous), Bentiaba lay approximately 10° further south at a latitude

analogous to that of modern Walvis Bay.


11

produced phosphatic sediments and specimens are not concen-

trated as in the Bench 19 Fauna.

In the case of the Bench 19 Bonebed, it is clear that a large

number of carcasses accumulated individually soon after death,

presenting a food source that attracted large numbers of mobile

scavengers, as evidenced by numerous associated shark teeth.

Decomposition was truncated by burial at varying stages, but

none of the carcasses remained on the seaf loor long enough

to transition beyond the mobile scavenger phase (sensu Smith

& Baco, 2003).

Stable carbon isotopes

Whole-body d13C values of consumer species largely ref lect

the carbon isotope values of primary producers on which they

ultimately feed, albeit with an ∼0–1‰ enrichment in 13C per

trophic step (Koch, 2007). The d13C of tooth enamel carbonate,

as opposed to whole-body values, is typically ∼9‰ more positive

relative to diet, but differences in the d13C of tooth enamel car-

bonate among marine species still largely reflects the d13C values

of primary producers (Koch, 2007). In marine settings the d13C

values of primary producers and particulate organic matter

become depleted with increasing distance from shore and

depth, possibly due to a more diverse group of primary producers

found in highly productive nearshore environments, including13C-enriched kelp beds and benthic macrophytes, relative to

offshore where 13C depleted marine phytoplankton are the

dominant primary producers (Clementz & Koch, 2001; Michener

& Kaufman, 2007). Clementz & Koch (2001) used these relation-

ships to examine the d13C values derived from tooth enamel

belonging to modern and recent marine mammals. They dem-

onstrated that a variety of niches were used in the California

upwelling zone and ref lected the trend of depleted d13C values

in primary producers located farther from shore (Fig. 8A).

Robbins et al. (2008) examined stable carbon isotopes from

a diverse sample of mosasaur teeth from numerous Upper

Cretaceous localities and interpreted the d13C values as

ref lecting feeding distance from shore and diving duration,

showing that the d13C values generally correlated with body

size. Schulp et al. (2013) examined five mosasaur taxa from

the type Maastrichtian section and found a strong correlation

between larger body size and lower d13C values.

The d13C values derived from tooth enamel (Fig. 6, Table 2)

from the Bench 19 Fauna show a general trend of decreasing

average d13C values with increasing body size, the pattern

previously documented in mosasaurs (Robbins et al., 2008;

Fig. 8. Comparison of d13C values of (A) modern primary producers and particulate organic matter (POM), (B) modern marine mammals (modified from

Clementz & Koch, 2001), and (C) tooth enamel carbonate of Bench 19 mosasaurs and plesiosaurs.


12

Schulp et al., 2013). Halisaurus and ‘P.’ ptychodon exhibit

enriched d13C values similar to modern sea otters and

ref lect a shallow marine feeding habitat at the coastline. d13C

values of Mosasaurus and Prognathodon are similar in value

(∼–12 to –8‰) to cetaceans and pinnipeds that feed in near-

shore California waters (Clementz & Koch, 2001). Prognathodon

teeth from Bentiaba have a much broader range of d13C values

(–14.8 to –5.2‰) than those reported from the Netherlands

(–13.6 to –12.3‰; Schulp et al., 2013). This greater range

may be a product of the larger sample size in this study, or it

may indicate that Bentiaba mosasaurs used a greater variety of hab-

itats or had broader diets. Average Globidens d13C values are more

depleted than expected for their body size, and are statistically in-

distinguishable from those of plesiosaurs. The d13C values of plesio-

saurs are consistent with foraging far from shore (Cicimurri &

Everhart, 2001), but this does not explain the values seen in Globi-

dens. The relatively depleted d13C values in Globidens are similar to

northern fur seals and elephant seals, whereas the values of the lat-

ter species are probably affected by routine dives >300 m in depth

(Clementz & Koch, 2001). Thus, Globidens d13C values can be

explained by foraging behaviour that involved increased dive time

(Biasatti, 2004) required by a diet of mollusks consistent with

reported stomach contents (Martin, 2007; Martin & Fox, 2007)

and with durophagus tooth morphology.

The d13C values of Bench 19 mosasaurs indicate that they

occupied a variety of niches as do modern marine mammals

in the upwelling waters off California. At least six mosasaur

taxa are represented in the Bench 19 Fauna, five of which were

analysed for d13C (Table 2). d13C values of tooth enamel, body

size and tooth-form disparity suggest niche differentiation in

preferred foraging zones ranging from nearshore to open ocean.

Although differentiation of foraging strategies would be

expected in a rich upwelling zone, the five Bench 19 mosasaur

taxa are associated in a short time horizon and a small area, and

are not spatially segregated as the d13C values predict.

The presence of Halisaurus and ‘P.’ ptychodon, both charac-

terised by relatively small body size and positive d13C values, is

consistent with the near-shore depositional environment of the

Bench 19 Bonebed. Likewise, the presence of the durophagous

mosasaur Globidens can be explained by the abundance of the

bivalve Inoceramus in the strata in which it is found; the

predator–prey relationship between these taxa is established

by Globidens stomach contents (Martin & Fox, 2007). The

stomach contents of Prognathodon kianda demonstrate the

association of three of the five mosasaur species in the Bench

19 Fauna as a function of predation or scavenging.

Prognathodon kianda shows a wide range of d13C values and

it had remains of three mosasaur individuals in its gut area. The

biological association of scavenging sharks with a diversity of

mosasaurs and plesiosaurs, coupled with the association of

three species of mosasaurs within the gut of a fourth, implies

that all Bench 19 mosasaurs were contemporary inhabitants

of this highly productive shallow, narrow shelf environment.

The association of mosasaur taxa with each other and with the

sharks that scavenged their flesh is indicative of the use of this

area as a foraging zone by individual taxa either as a relatively

permanent home range or as an opportunistic feeding ground.

Conclusions

The Bench 19 Bonebed at Bentiaba, Angola, formed in a shallow,

narrow, highly productive shelf environment characterised by

relatively low sedimentation rates compared to bone input.

The Bench 19 Bonebed represents a residual accumulation of

instantaneous biological interactions that introduced carcasses

into the sedimentary system where the animals lived, at the

time they died, and in that sense, it is attritional. Although

we cannot constrain the amount of time represented to less

than 240,000 years, we interpret the Bench 19 Fauna as an

accurate representation of the Late Cretaceous marine amniote

ecosystem in offshore Angola. The Bench 19 Bonebed contains

the richest, temporally constrained and intimately associated

assemblage of mosasaurs yet known from the fossil record,

and as such it provides an unparalleled window into the ecolog-

ical relationships of Late Cretaceous marine ecosystems on the

southwest African coast of the widening South Atlantic Ocean.

Acknowledgements

This publication results from Projecto PaleoAngola, an international

cooperative research effort among the contributing authors and their

institutions, funded by the National Geographic Society, the Petro-

leum Research Fund of the American Chemical Society, Sonangol E.

P., Esso Angola, Fundaç~ao Vida of Angola, LS Films, Maersk, Damco,

Safmarine, ISEM at SMU, the Royal Dutch Embassy in Luanda, TAPAir-

lines, Royal Dutch Airlines and the Saurus Institute. Detrital zircons

were analysed courtesy of the ExxonMobil Upstream Research Com-

pany. We thank Margarida Ventura and Andre Buta Neto for provid-

ing our team with help in the field. Tako and Henriette Koning

provided valuable support and friendship in Angola. We are

grateful to Diana Vineyard for her facilitation of this study.

Roy Beavers generously aided with SEM and mineral analysis.

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Geological setting and paleoecology of the Upper